The present invention relates to a technique for recording a signal on a flat recording medium, or more in particular to a method and apparatus for recording a signal with an improved density and a tracking technique for reproduction.
An example of a flat recording medium is one used in the technique known as the video disc player which utilizes light or change in electrostatic capacity in reproduction. The present invention is applicable to both types of method. Especially, it is an art suitably applied to a system using a recording medium without any guide slot in the recording track.
An example of the reproducing apparatus using light for reproduction is shown in FIG. 1. In the drawing under consideration, the essential parts are illustrated schematically. Reference numeral 1 shows a light source, numeral 2 a half mirror, numeral 3 a lens, and numeral 4 a recording medium in the form of disc. The surface of the disc 4 where information is recorded is comprised of a reflection surface coated with aluminum or the like. The information such as a video signal or an audio signal generally takes the form of a modulated carrier wave recorded as a repetition of pits along a spiral track, i.e., spatial frequency. A pit is represented by a small dent on the reflection surface which is as large as, for example, 0.4 .mu.m wide, 0.8 .mu.m long and 0.15 .mu.m deep. The pit width is called a track width, and the distance between adjacent tracks a track pitch. The track pitch is generally set at about 1.7 .mu.m in the conventional system.
The light from the laser light source is condensed at the information recording surface of the disc 4 by the lens 3 through the half mirror 2. Part of the light reflected on the information recording surface is reflected on the half mirror and condensed at a light receiver 5 such as a photo-diode. The amount of light reflected depends on the absence or presence of a pit. The change in the amount of light applied to the light receiver 5 is taken out as an electrical signal and after being amplified by the amplifier 6, processed as required. Such a reproduction system is disclosed in detail, for example, in Journal of the SMPTE, July 1974 Volume 83 P. 564 to 566 "The Philips `VLP` system" by K. Compaan et al, P. 567 to 571 "Signal processing in the Philips `VLP` system" by W. van den Bussche et al, P. 572 to 575 "The optical scanning system of the Philips `VLP` record player" by G. Bouwhis et al, and P. 576 to 579 "Control mechanisms in the Philips `VLP` record player" by P. J. M. Janssen.
FIG. 2a shows an enlarged sectional view of the disc 4 cut away radially, in which a mirror reflection surface is shown. Actually, this surface is coated with a plastic protective film, which is not shown in the drawing. Numerals 8 and 8' show pit positions for each track. The dents as shown are formed in the presence of pit. The dashed line curve 10 shows a profile of the light beam spot, and numerals 11 and 11' feet at which the amplitude of light becomes zero. These points are called null points and the distance therebetween a null width.
The disc makes 30 rotations in one second, in accordance to which the light spot scans the spiral track. In accordance with the presence or absence of the pit, the amount of light passed from the disc to the lens changes. This change is taken out as an electrical signal. As well known, in the absence of a spot on the pit, that is, in the presence of a spot on the mirror surface, all the reflected light returns to the lens. In the presence of a spot on the pit, however, the return light is directed both to the right and left by diffraction, and therefore substantially no light returns to the lens. In order to assure an effective diffraction, the pit depth is set at about one fourth of the wavelength of the laser beam.
A rotary disc poses a problem of eccentricity. Generally, an eccentricity of about 100 .mu.m occurs. This corresponds to about 60 tracks in view of the track pitch of 1.7 .mu.m. In FIG. 1, it is well known that the lens or the like is vibrated laterally to follow the lateral displacement due to the eccentricity. The problem is the means for detecting the positional displacement of a desired track, for which several conventional methods are suggested, all based on the principle described below.
In the graph of FIG. 2b containing the curve 12, the abscissa represents the radial coordinate of the disc around the spot and the ordinate the detected information output of the light receiver 5 shown in FIG. 1. As seen from this drawing, when the spot center is positioned exactly on the track, the maximum information output is produced, while it is positioned between tracks, the information output is zero. Thus, the magnitude of the information output (the envelope amplitude of the high frequency signal or the magnitude of the DC portion representing the average reduction in return light) presents an index showing the degree of coincidence between the track center and spot center, although the right and left polarities of the displacement are not known. A conventional method to overcome this problem is what is called the "wobbling method" in which the spot position is vibrated finely at about .+-.0.2 .mu.m. The right and left polarities are determined by the resulting direction of the gradient of the change in the information output.
In another conventional method for determining the polarity of displacement, a plurality of light beam spots exclusive for track position detection are used in addition to the main light beam spot.
Anyway, in the conventional methods, the fact is utilized that when the spot is displaced from the track center, the information output is reduced, as shown in the curve 12 of FIG. 2b. Since the reduction in the information output is laterally symmetric, a complicated optical system is inconveniently required for determination of the polarity.
Another problem is that of recording density. In the case of FIG. 2a, the null width of the spot is substantially equal to the track pitch. The result of halving this track pitch is shown in FIGS. 3a and 3b. As seen from 13 of FIG. 3b, the information output is substantially constant regardless of the positional displacement of the spot center. In other words, as long as the spot is situated on the desired track center, only the desired track information is capable of being picked up. When the spot is displaced from the track center, on the other hand, the desired track information is reduced in accordance with the magnitude of displacement on the one hand and the adjacent track information is increased in accordance with the magnitude of displacement, resulting in a substantially constant total information output.
Thus, it is impossible to use the conventional track position error detecting system which relies on the fact that the information output is reduced in the bottom between tracks. In the conventional systems, therefore, the limitation is posed in which the recording density must be set in about the relation between the spot width and the track pitch as shown in FIG. 2a.
Generally, the null width N of the light beam spot is substantially equal to the wavelength of the laser beam divided by the numerical aperture of the lens. It is technically very difficult to reduce this value. Therefore, it is desired to achieve the minimum recording pitch at a constant null width of the spot. In the conventional systems, however, the principle of the track position error detecting means poses a restriction almost as shown in FIG. 2a.
In one of the conventional methods which uses no light for reading an information signal, a pit array similar to the method using an optical system is provided on a conductive disc, and the presence or absence of the pit is read as a change in electrostatic capacity. Such a method is disclosed, for instance, in RCA Review Volume 39 No. 1 March 1978, P. 7 to 13 "The RCA `Selectavision` Video Disc System" by D. S. McCoy. In this method, a continuous slot or groove is formed along with the S-shaped pit array, so that a pit array modulated by the information signal is formed in the S-shaped continuous slot. For this reason, a signal-reading stylus traces the inside of the slot as in the conventional audio disc record thus attaining an automatic tracking. In this system having a tracking slot or groove, it is impossible to reproduce a still image after recording a video signal, that is, it is impossible to trace the same track repetitively by turning the stylus to the same track for each rotation of the disc.
In order to meet the requirement for still image reproduction, a recording-reproduction method has been developed in which a pit array lacking a stylus guide slot is formed and the change in electrostatic capacity is utilized. In this system lacking the slot for guiding the signal reading stylus, a pilot pit array carrying the pilot signal for tracking is provided between adjacent pit arrays for the main information signal. In the reproduction system, this pilot signal is read and any displacement of tracking is thus detected for the purpose of tracking by an automatic control through a tracking servo system. The advantages of this system are that as in the optical system, reproduction of a still image and reproduction at a speed different from the ordinary speed is easy and that the fact that the pressure of the stylus for signal reading is maintained comparatively low lengthens the life of the disc and stylus.
In spite of this advantage, this system has the shortcoming that the recording operation requires two light beams including the main light beam for main signal recording and the pilot light beam exclusive to pilot signal recording. The pilot beam is capable of being produced by separation and synthesis from the main beam by using a half mirror. In view of the requirement for high recording density, however, the distance between the main beam and pilot beam must be maintained at high accuracy of about 0.7 .mu.m.+-.0.1 .mu.m on the disc. The mounting of the half mirror makes it very difficult to maintain such an accuracy in recording process. A slight temperature change, vibration or change with time often results in an error and a reduced recording yield, thus posing a problem in the industrial application thereof. Also, a photo-resist photo-sensitive film process is required in the course of recording and therefore inclusion of dust and dirt often leads to a defect on the disc.